The present invention relates to compositions having photocatalytic self-cleaning properties for use as translucent coatings on construction material surfaces, and, more particularly, the present invention relates such improved compositions having high de-HNO3 activity.
In the field of buildings and coatings, the pollution of the environment raises a serious problem of contamination of exterior materials for buildings and outdoor buildings. Dust and particles floating in the air deposit on the roof and the outer wall of buildings in fine weather. Upon exposure to rainfall, the deposits flow together with rainwater and flow down along the outer wall of the building. As a result, the contaminant adheres along the course of the rainwater. As the surface dries, soil appears in a stripe pattern.
To solve at least in part this problem, it has already been proposed to deposit a coating on construction material surfaces. Alternatively, said coating furthermore exhibits photocatalytic self-cleaning properties towards atmospheric contaminants. Thus, titanium oxide photocatalytic coatings are disclosed in EP 0901 991, WO 97/07069, WO 97/10186 and WO 98/41480.
Ultraviolet radiation (for example from UV light) to a semiconductor, such as titanium oxide, generates electrons having a strong reduction activity and positive holes having a strong oxidation activity which can ultimately initiate the degradation of harmful organic compounds into harmless substances. Typical atmospheric contaminants are for example, nitrogen oxides, ozone and organic pollutants adsorbed on the coated surface of the materials. This is particularly advantageous in built-up areas, for example, in city streets, where the concentration of organic contaminants may be relatively high, especially in intense sunlight, but where the available surface area of materials is also relatively high.
However, one problem associated with so-formed oxidized species, like HNO3, formed from the reaction of NO2 and NO with TiO2/UV light in the presence of water and oxygen, is their absorption on the coated surface of the material which may then over time cause problems with difficult-to-remove stains and/or corrosion.
Accordingly, there is still a need for a coating composition having a significant improvement in decontamination properties, non staining ability and outstanding durability over prior coatings.
Surprisingly, the inventors have discovered that such a purpose could be efficiently achieved by a specific composition for use as a coating.
Accordingly, an object of the present invention is to provide a composition which, when applied as a coating on a surface of a material, exhibits improved NOx and optionally VOCx (i.e., Volatile Organic Content, like xylene and benzene) removing properties.
Another object of the present invention is to provide a composition which can impart such properties without sacrificing the translucency of the coating.
Still another object of the present invention is to provide a composition which, when applied as a coating on the surface of a material, can easily release the contaminant therefrom, in particular by rainfall or by washing with water. Specifically, the composition, when applied to the surface of a substrate to form a thin film, enables a contaminant or derivative thereof adhered onto the surface to be easily washed away with water.